Sulfur is an objectionable element that is typically found in fossil fuels, where it occurs both as inorganic sulfur, such as pyritic sulfur, and as organic sulfur, such as a sulfur atom or moiety present in a wide variety of hydrocarbon molecules, including for example, mercaptans, disulfides, sulfones, thiols, thioethers, thiophenes, and other more complex forms. Crude oils can typically contain, for example, amounts of sulfur up to 5 wt % or more.
The presence of sulfur in fossil fuels has been correlated with the corrosion of pipeline, pumping, and refining equipment, and with the premature breakdown of combustion engines. Sulfur also contaminates or poisons many catalysts which are used in the refining and combustion of fossil fuels. Moreover, the atmospheric emission of sulfur combustion products, such as sulfur dioxide, leads to the form of acid deposition known as acid rain. Acid rain has lasting deleterious effects on aquatic and forest ecosystems, as well as some agricultural areas located downwind of combustion facilities. To counter these problems, several methods for desulfurizing fossil fuels, either prior to or immediately after combustion, have been developed.
One recently developed technique for desulfurizing fossil fuels is known as biodesulfurization (BDS). BDS is generally described as the harnessing of metabolic processes of suitable bacteria to the desulfurization of fossil fuels. Thus, BDS typically involves mild conditions, such as ambient or physiological, and does not involve the extremes of temperature and pressure. Kilbane, U.S. Pat. No. 5,104,801 describes one such process wherein a mutant Rhodococcus strain ATCC No. 53968 selectively cleaves the C--S bond in organic carbonaceous materials. The efficiency of the BDS process can be improved by employing an emulsion or microemulsion. See copending application Ser. No. 07/897,314, now U.S. Pat. No. 5,358,970 incorporated herein by reference.
Processes, such as the above process, employ multiple liquid phases or result in the formation of emulsions or microemulsions. It is often difficult to resolve or separate emulsions and microemulsions employing conventional apparatus such as separators, coalescensors or electrical precipitators. Capillary cross-flow membranes or filters, such as those employed herein, are conventionally employed in solid-liquid separations. Mawson et al., Australasian Biotechnology, 3:348-352 (1993).